Presence detector having automatic digital tuning

ABSTRACT

Apparatus for determining whether a movable body, such as a railway car traveling along a track, is present in a preselected zone of interest along its path of travel. In railway applications for detecting the presence of a car over a switch, for example, an inductive loop is laid along the track and forms a part of the tank circuit of an oscillator. The resulting signal is mixed with the output signal from a variable frequency reference oscillator to provide an intermediate frequency signal that is subject to frequency variations as the loop inductance changes. To correct for such variations, an automatic digital tuning arrangement is provided for the reference oscillator to maintain the IF signal at a normal frequency within a predetermined range within which the intermediate frequency is permitted to vary in response to changing environmental conditions. The response of the digital tuner is sufficiently rapid to correct gradual variations in frequency characteristic of changing environmental conditions, but is purposely limited in order to be incapable of making the correction in the event of a sudden frequency variation caused by the presence of a car. When this occurs, the intermediate frequency shifts outside of the normal range and the apparatus assumes an output condition indicative of the presence of the car. Once the car is detected, the automatic tuner is disabled to positively preclude frequency correction until the car is no longer sensed by the loop.

allinger et at,

n] amen 1 June 25, 1974 PRESENCE DETECTOR HAVING AUTOMATIC DIGITAL E [75] Inventors: Forrest II. Ballinger, Grain Valley; Robert E. Heggestad, Blue Springs; Leland W. Olmsted, Gladstone, all of Mo.

[73] Assignee: Harmon Industries, Inc., Grain Valley, Mo.

[22] Filed: Sept. 27, 1972 [21] Appl. No.: 292,737

[52] US. Cl 340/258 C, 246/249, 331/1 A, 340/38 L [51] Int. Cl. G081) 13/26 [58] Field of Search 340/258 C, 258 B, 38 L; 331/1 A, 65; 246/249; 325/422; 328/141 [56] References Cited UNITED STATES PATENTS 2,421,771 6/1947 Browning 340/258 C 3,185,938 5/1965 Pelosi 1331/] A 3,346,856 10/1967 Doble et al 340/38 L 3,364,465 1/1968 Prucha 340/38 L 3,488,605 1/1970 Schwartz 331/1 A 3,688,308 8/1972 Makoto et al 340/38 L 3,710,274 1/1973 Basse et al. 331/1 A Primary Examiner-John W. Caldwell Assistant Examiner-Glen R. Swann, Ill Attorney, Agent, or Firm-D. A. N. Chase Apparatus for determining whether a movable body, such as a railway car traveling along a track, is present in a preselected zone of interest along its path of travel. In railway applications for detecting the presence of a car over a switch, for example, an inductive loop is laid along the track and forms a part of the tank circuit of an oscillator. The resulting signal is mixed with the output signal from a variable frequency reference oscillator to provide an intermediate frequency signal that is subject to frequency variations as the loop inductance changes. To correct for such variations, an automatic digital tuning arrangement is provided for the reference oscillator to maintain the IF signal at a normal frequency within a predetermined range within which the intermediate frequency is permitted to vary in response to changing environmental conditions. The response of the digital tuner is sufficiently rapid to correct gradual variations in frequency characteristic of changing environmental conditions, but is purposely limited in order to be incapable of making the correction in the event of a sudden frequency variation caused by the presence of a car. When this occurs, the intermediate frequency shifts outside of the normal range and the apparatus assumes an output condition indicative of the presence of the car. Once the car is detected, the automatic tuner is disabled to positively preclude frequency correction until the car is no longer sensed by the loop.

16 Claims, 5 Drawingl igures DECADE LOOP TUNlNG CAPAClTOR 3a 34 Soon: 7 1 36 ER eAg t A ss AMP L y v 4O 1 REF. Z6 44- METER osc. FlLT-ER 38 at s2.- a? SW ,64 SU'PPLY 46 7 CONTROL la 70 i I P'L/ 4a f \w I W 156 r 58, I50 l PERIOD DlGlTAL film X i g f2 COUNTER COMPARATOR ll 1 1 62 1 I INCREMENTAL UP/MWN TUNER CONTROL 1 PRESENCE nnrncron A r 1 DIGITAL t;

This invention relates to improvements in apparatus for detecting the presence of a movable metallic body and, in particular, to an improved presence detector having automatic digital correction tuning for preventing false presence indications in response to changing environmental conditions.

In the railroad industry, presence detectors enjoy widespread use in a number of applications including the protection of remotely controlled track switches. For example, switching in classification yards involves the routing of freight cars along selected tracks in accordance with their ultimate destination, and it is not uncommon for one hundred or more of such switches to be employed in one yard for this purpose. Instances may occur Where the controller inadvertently throws a switch too late, which obviously could cause the car to overturn unless the switch control is automatically disabled during the time that the car passes over the switch. Accordingly, it has been the practice to place an inductive loop along the track at each switch location, such loop forming a part of the tank circuit of an oscillator which suddenly changes in frequency when the metallic body of the car is present. This frequency change is then utilized as an indication of the presence of the car and is employed to disable the switch control until the car has passed.

However, it has been found that changing environmental conditions are also capable of changing the frequency of the loop oscillator. Moisture fromrrain and snow, for example, can induce a marked change in frequency although this change will occur over a relatively long time period as compared with the sudden change induced by the presence of a car. Nonetheless, this requires that the frequency of the oscillator be adjusted periodically and particularly after a heavy rain or snow. If this is not done, many of the presence detectors in the classification yard may all indicate that a car is present after a rain which, of course, is a severe problem to operating personnel.

Although operating personnel in a classification yard may certainly adjust the presence detectors to compensate for a changing environment, this is not done as a practical matter and the detectors are permitted to drift in frequency. This naturally suggests that the equipment should be provided with an automatic frequency control, but it should be understood that the correction must be made only in response to relatively slow changes in frequency characteristic of changing environmental conditions. A correction must not be made in response to the presence of a car even if delayed in time, since there is always the possibility that the car will be purposely stopped over the switch for a relatively long period. Accordingly, a problem is presented in that the frequency correction must be permanent, yet the apparatus must be able to distinguish between a frequency variation caused by the environment and a variation resulting from a passing car.

It is, therefore, the primary object of the present invention to provide apparatus for detecting the presence of a movable metallic body, such as a railway car or the like, wherein automatic compensation for changing environmental conditions is employed to prevent the apparatus from falsely indicating that a car is present.

As a corollary to the foregoing object, it is an important aim of this invention to provide apparatus as aforesaid in which the frequency of an oscillatory electrical signal varies in accordance with changing environmental conditions as well as the presence of a car in a zone of interest, and wherein the apparatus automatically corrects the frequency in response to relatively slow variations characteristic of changing environmental conditions but is insensitive to the sudden variation produced as the car passes through the zone.

It is another important object of this invention to provide apparatus as aforesaid in which such frequency correction will not occur in response to the sudden variation produced as the car enters the zone, nor subsequently until the car leaves the zone even if it is permitted to remain stationary therein for a indefinite period of time.

It is still another object of this invention to provide apparatus as aforesaid in which the automatic tuning function is accomplished by digital means in order to provide ultimate reliability and effect a permanent frequency correction at a controlled rate.

Furthermore, it is an important object of this invention to provide a digital frequency correction system which effects a permanent change in the capacitive or inductive components of a tuned circuit to correct the resonate frequency thereof.

In the drawings:

FIG. 1 is a simplified block diagram of the presence detector;

FIGS. 2a and 2b comprise an expanded block and logic diagram of the detector apparatus;

FIG. 3 is a timing diagram illustrating the count, store and reset functions; and

FIG. 4 is a truth table associated with the determination of frequency by digital means.

THE DETECTOR Referring initially to FIG. 1, a sensing element in the form of an inductive loop 10 is shown located adjacent a railroad track 12 at a zone thereof where a track switch 14 is located. The switch is shown to the position diverging from the track 12 so that a railway car (not shown) will be directed onto the diverging track 16. Track switches are commonly thrown by electrically, pneumatically or hydraulically operated switch controls such as illustrated at 18. In a classification yard, for example, the switch control 18 would commonly be remotely operated by a controller in charge of positioning the switches throughout the yard to classify the cars in terms of their ultimate destinations as is well known in the railroad industry.

The loop 10 provides a tuning inductance for the loop oscillator 20, in conjunction with a decade tuning capacitor 22. The loop 10 may comprise insulated wire laid alongside the railroad ties throughout the length of the zone of interest, with the crossover at each end of the loop being provided by running the wire across the track beneath the rails and between the ties. Since in this illustrative example the loop 10 and associated circuitry to be described is employed for the purpose of protecting a switch, the region of interest is the area along the track 12 at the switch 14 having a length approximately equal to a single freight car. Although various frequencies may be used, it will be assumed for purposes of illustration. in this specification that the loop oscillator 20 is tuned by the capacitor 22 to a frequency of 90.4 Kl-I2.

A variable frequency reference oscillator 24 has a tank circuit illustrated at 26 and delivers its output to a mixer 28. It will be assumed for purposes of illustration that the frequency of the reference oscillator is 80.5 Kl-lz when the loop oscillator 20 is at a frequency of 90.4 KHZ. The loop oscillator output is also fed to the mixer 28, thus the mixer combines the signals from the two oscillators 20 and 24 to produce an output havin g a frequency equal to the difference between the frequencies f the oscillators (normally 9.9 KHZ). This difference frequency (intermediate frequency) is fed to a band pass filter 30 which has a band width of 500 Hz. The center frequency of the pass band is approximately 9.9 KI-Iz, thus the IF signal is permitted to vary 50 Hz before it is severely attenuated by the filter 30.

The IF signal drives an amplifier 32 whose output is fed to a bridge rectifier 34. The rectified DC output is employed to operate an electromechanical relay having a coil 36 which is maintained normally energized. The contacts 38 of the relay are shown held closed by the energized coil 36 to maintain electrical continuity between a pair of leads 40 and 42. These leads are shown extending to the switch control 18 and would, for example, be in series with the power circuit to the switch control 18 so that, upon opening of the relaycontacts 38, the switch control 18 is disabled until such time that the relay coil 36 is again energized.

For test and alignment purposes, a meter filter 44 is shown that also receives the output from amplifier 32' before rectification. A volt meter 46 is connected to a negative DC supply terminal 48 and the movable pole 50 of a rotary switch. Accordingly, the meter 46 may be employed at the track location to check the DC supply voltage, the output of amplifier 32, or the voltage across the relay coil 36 as is clear from the switch positions indicated in the drawing. In normal operation, the pole 50 of the rotary switch is in the off position.

It is now instructive to consider the basic manner in which the above described detector operates, before proceeding with a description of the automatic digital tuning. The 90.4 Kl-Iz frequency of the signal produced by the loop oscillator 20 is the frequency under normal conditions as set by the tuning capacitor 22, and corresponds to an IF of 9.9 KHz (the normal frequency at approximately the center of the 500 Hz pass band). However, the loop oscillator 20 is subject to variations in its output frequency caused by changing environmental conditions in the zone of interest and, of course, the presence of a railway car in such zone. The change in inductance of the loop is relatively slow in the case of environmental conditions, but sudden in response to a railway car entering or leaving the proximity of the loop 10. Accordingly, the frequency of the IF signal is permitted to vary from the normal frequency so long as it remains in the pass band of the filter 30, and it will remain therein assuming that the tuning capacitor 22 is periodically readjusted to compensate for changing environmental conditions. However, a metallic body (a railway car in the exemplary application of the apparatus illustrated herein) sensed by the loop 10 causes the frequency of the IF signal to shift outside of the pass band, resulting in a loss of output at the bridge rectifier 34 to de-energize the relay coil 36 and open the contacts 38. This disables the switch control 18 until such time that the car leaves the switch locations so that the switch cannot be thrown while it is beneath a passing car.

In the foregoing discussion it was assumed that the tuning capacitor 22 would be periodically adjusted to compensate for changing environmental conditions. Otherwise, the apparatus would produce a false indication of the presence of a car once the environment effected a shift of the frequency of the IF signal outside of the pass band of filter 30. The remainder of this specification will be primarily directed to the digital tuning feature of the present invention which automatically compensates for changing environmental conditions to eliminate the need to retune the loop oscillator 20 as weather changes occur.

AUTOMATIC DIGITAL TUNING As indicated by the lead 52 in FIG. 1, the IF signal is also fed to a period counter 54 together with the output from a 1 MHz crystal oscillator 56. Through digital counting the period counter 54 determines the number of cycles of the output of the oscillator 56 present in successive time intervals in order to continuously monitor the frequency of the IF signal. This information or count is fed to a digital comparator 58 where the count is compared with a numerical value representing the normal frequency of the IF signal. The comparator out put is then fed to an up/down control 60 that, in turn, drives an incremental tuner 62. In accordance with the output from the comparator 58, the tuner 62 is commanded to increase or decrease capacitance or maintain the existing value of capacitance so that the reference oscillator 24 will follow any variations in the output frequency of the loop oscillator 20. A lead 64 is illustrated connecting the tuner 62 with the tank circuit 26 of the oscillator 24.

It should be noted, however, that the digital circuitry of the present invention is not permitted to make an instantaneous correction in the event that the IF signal drifts in frequency. This is illustrated in the simplified block diagram of FIG. 1, where it may be seen that a five second pulse generator 66 is provided having an output along a lead 68 connected to the up/down control 60. The up/down control will effect the automatic tuning function only in response to a pulse from a generator 66, thus a change in the total capacitance in the tank circuit 26 can be made only once each 5 seconds. This prevents the automatic tuner from correcting the frequency of the IF signal in response to a sudden frequency variation, such as would be encountered when a railway car enters the field of the loop 10. A pair of power leads 70 to the generator 66 extend from the output of the bridge rectifier 34, thus the pulse generator 66 is rendered inoperative whenever the apparatus responds to the presence of a car. Accordingly, any correction in the signal frequency cannot be made until such time that the car leaves the proximity of the loop 10. This positively prevents the apparatus from correcting the frequency in the event that the car should stop over the track switch for a long period of time. If the pulse generator 66 were permitted to operate continuously, then in sufficient time the signal could be restored to a frequency within the 500 Hz pass band and the relay coil 36 would again be energized to falsely indicate that the car had passed the switch.

The digital tuning apparatus is shown in detail in FIGS. 2a and 2b. The IF signal appearing on the lead 52 (illustrated by the wave form 74) is fed to a shaper 72 which delivers a square wave output illustrated by the train of pulses 76. The square wave is then fed to a divide by ten network 78 which drives the clock inputs C of a pair of type D flip-flops 80 and 82. The network 78 serves a prescaling function and delivers one pulse at its output for every ten pulses delivered to its input by the shaper 72. Accordingly, the pulse frequency at the clock inputs C of the flip-flops 80 and 82 would normally be approximately 0.99 Kl-Iz. Period counter sequencing is achieved by the two flip-flops 30 and 82 in conjunction with a 3-input NAND gate 84 and a 2-input NAND gate 86. The 6 output of flip-flop 80 is connected to the data input D of the flip-flop 82 and one of the inputs of each of the NAND gates 84 and 86. The Q output of flip-flop 82 is connected to the data input D of the flip-flop 80 and the second input of the NAND gate 84. The Q output of the flip-flop 82 is connected to the second input of the NAND gate 86, and a lead 88 identified by the legend store extends on from this output to later portions of the logic circuitry as will be discussed.

The l MHZ crystal oscillator 56 serves as a time base and has its output connected to the third input of the NAND gate 84. The output of NAND gate 84 is identified by the legend count and is fed to a divide by six network 92. The timing signals at the output of NAND gate 84 are illustrated at 94 where it may be appreciated that each open condition of the gate 84 permits the 1 MHz output from the oscillator 56 to be conducted to the divide by six network 92. An inverter 96 receives the output of the NAND gate 86 and executes a reset function along a lead 98 as the legend in FIG. 2a indicates. The output of the divide by six network 92 appears along a lead 100 and is fed to a four-stage binary counter 102. The series of timing signals reaching the counter 102 upon each operation of the NAND gate 84 occupy a time duration of approximately I msec., there being a period of approximately 3 msec. between each series as is illustrated in FIG. 3. The number of timing signals (corresponding to cycles of the l MHz oscillator output) in each series, however, is reduced by a factor of six by the divide by six network 92; therefore, the counter 102 will deliver a count indicating that the frequency of the IF signal is at the normal frequency to an accuracy of approximately 5 I-Iz, as will be discussed more fully hereinafter with respect to the table shown in FIG. 4. Accordingly, due to the scaling effected by the various networks, the period during which timing signals are counted by the counter 102 has a time duration of approximately times the period of the intermediate frequency (approximately 0.1 msec. at 9.9 KI-llz).

The output of the four-stage binary counter 102 is fed to one set of inputs of the digital comparator 58. A second set of inputs are hard wired as illustrated to place a l-bit on the input identified 8 and a O-bit on the inputs identified 1, 2 and 4. When the count from the counter 102 is eight, both of the output leads 104 and 106 of the comparator 58 are at the high logic level. If the count from counter 102 is less than eight, the lead 104 is at the high level and the lead 106 is at the low level. Conversely, if the count is greater than eight, the lead 104 is low and the lead 106 is high. (It should be understood that two voltage levels are employed in the present invention in accordance with binary logic, and are referred to herein as the low level or O-bit and the high level or one-bit.)

The leads 104 and 106 are connected with the data inputs D of a pair of type D flip-flops 108 and 110 respectively. The Q output of flip-flop 108 is connected to one input of a 2-input NAND gate 112; similarly, the Q output of the flip-flop is connected to one input of a 2-input NAND gate 114. The store lead 88 is connected to the clock inputs C of both of the flip-flops 108 and 110. The gates 112 and 116 are controlled by the five second pulse generator 66, which comprises a unijunction transistor oscillator 116 that delivers its output to a monostable multivibrator or one shot 118. The time-spaced pulses produced at the output of the one shot 118 are illustrated at 120 and are fed to the second inputs of the gates 112 and 114.

The two flip-flops 100 and 110 comprise a two-bit storage register that receives the output information from the digital comparator 58. The two NAND gates 112 and 1 1 1 are responsible for the function of the up/- down control 60 illustrated in the block diagram of FIG. 1. The turner 62 is controlled by the outputs of these gates 112 and 114 in order to change the total capacitance in the tank circuit 26 of the reference oscillator 24.

More particularly, the tuner 62 comprises a four-bit up/down decade counter 122 having a down clock or down counting input 124 and an up clock or up counting input 126. Four digital leads 128 extend from the outputs of the four sections of the counter 122 and are connected with the bases of four switching transistors 130 respectively. If a one-bit is present on a particular lead 128, a voltage at an amplified level is delivered to the corresponding transistor 130 to switch the latter to its conductive state. It should be noted that the collector of each transistor 130 is connected to one plate of a capacitor 132, the other plate thereof being connected to the lead 64. Accordingly, any one of the in capacitors 132 or any combination thereof representing a decimal number up to and including nine may be connected between the lead 64 and common ground as indicated by the symbol in order to place additional capacitance jn the tank circuit 26.

The counter 122 has a borrow output 13 and a carry output 136 driving the down and up counting inputs, respectively, of a four-bit up/down binary counter 138. Four digital leads 140 extend from the four sections of the counter 138 to respective transistor switches 142, and four capacitors 144 are connected between the collectors of respective transistors 142 and the lead 64. Accordingly, since the counter 122 has ten states and the counter 138 is binary hexadecimal (l6 states), the total incremental capacity of the two counters is steps or states. Furthermore, the capacitors 132 and 144 are progressively larger in value from left to right so that eight different values of capacitance are provided in the incremental tuner 62.

OPERATION As discussed hereinabove, the network 78 responds to the pulse train 76 from the shaper 72 and provides a pulse width at its output that is increased by a factor of 10. The output of the network 78 is illustrated in the first or top graph of the timing diagram of FIG. 3. Each pulse cycle (with an IF of 9.9 Kl-Iz) has a period of approximately l.0 msec. or ten times the length of the period of the IF signal. The first pulse period in the output from network 78 is identified in FIG. 3 by the legend count, the next period is identified settle, the third period is identified dead, and the final period is identified reset. This constitutes one complete sequence of operation.

The period counter sequencing may be further understood from the 9ther graphs in FIG. 3. The second graph shows the Q output of flip-flop 80, the third graph is the Q output of flip-flop 82, and the fourth graph is the Q output of flip-flop 82. The remaining graphs are identified count and reset and represent the outputs from the NAND gate 84 and the inventer 96 respectively. It may be appreciated that when both the output of the flip-flop 80 and the Q output of the flip-flop 82 are at the high logic level, the output from the 1 MHz crystal oscillator 56 will be delivered at the output of the NAND gate 84 in inverted form. This is clearly portrayed in FIG. 3 where it may be seen that the 1 MHz oscillations are delivered by the NAND gate 84 during the first pulse period of the output from the network 78. Accordingly, the pulse output from the network 78 serves as a clock to operate the flip-flops 80 and 82 in order to execute the period counter sequencing.

During the counting phase of the sequencing cycle, the oscillations of the l MHZ crystal oscillator 56 are counted by the binary counter 102 after the divide by six network 92 reduces the number of oscillations by a factor of six. Accordingly, the information reaching the counter 102 via lead 100 is in the nature of a series of timing signals indicative of the period (and hence the frequency) of the IF signal.

Now referring particularly to FIG. 4, it may be seen that a count equal to the decimal number 8 corresponds to an IF of from 9,872 to 9,921 Hz. This is the normal frequency of the IF signal referred to hereinabove, thus it is to be understood that the term normal frequency as used herein may represent a discrete frequency or a relatively narrow band of frequencies as illustrated in FIG. 4. The width of this narrow band is determined by the resolution of the system and depends entirely upon the accuracy that a particular application may require. In the exemplary system illustrated herein, the normal frequency of the IF signal covers a band having a width of approximately 49 Hz. The total count range set forth in the table of FIG. 4 refers to the number of 1 MHz pulses at the output of the NAND gate 84' which are delivered to the divide by six network 92.

It should be noted at this juncture that, although comparison information will appear on the leads 104 and 106 from the output of the comparator 58 throughout the counting phase, such information is not stored in the flip-flops 108 and 110 until a pulse is received by the clock inputs of these flip-flops. Since the binary counter has a decimal counting capability of 0 through and on the order of 170 pulses from the divide by six network 92 will be counted during each counting operation, the output from the digital comparator 58 is not stored until the end of a settling period defined by the second pulse period of the output of the divide by ten network 78. This is clear in FIG. 3 where it may be seen that the command to store is a positive going pulse from the Q output of flip-flop 82 having a leading edge 146. The command to store is conducted along lead 88 to the clock inputs of the flip-flops 108 and 110, thereby causing their Q outputs to follow the information present from the comparator at the data inputs D. If the leads 104 and 106 are both at the high logic level, then the Q outputs of the flip-flops 108 and 110 will be at the low level and no frequency correction will be made. If either of the leads 104 or 106 is at the low level, then the Q output of the corresponding flip-flop 108 or 110 will assume the high logic level to open the succeeding NAND gate 112 or 114.

It will now be assumed for purposes of illustration 7 that the final state of the output of the counter 102 corresponds to the decimal number 7. This is at the beginning of the settling period of the operational cycle depicted in FIG. 3. The digital comparator 58 makes a comparison of the count of seven with the numerical value eight and indicates that the frequency of the IF signal has drifted upward. The lead 104 is now at the low logic level rather than high, thus the Q output of flip-flop 108 assumes the high logic level at the end of the settling period in response to the leading edge 146 of the clock pulse constituting the store command. However, the up/down control 60 constituting the gates 112 and 114 do not respond to the information stored in the register 108-1 10 until an output pulse 120 is delivered by the one shot 118. Thus, the information now stored in the flip-flop 108 may remain therein without activating the incremental tuner 62 for as long as approximately 5 seconds. When a pulse 120 is produced, the output of the NAND gate 112 shifts to the low logic level since now both of its inputs are high. The counting inputs 124 and 126 of the counter 122 are activated in response to a change in the logic level from low to high, thus in this instance the up input 126 is activated and the preexisting count in the counters 122 and 138 is increased by one. This changes by one increment the capacitance added to the reference oscillator tank 26 by the capacitors 132 and/or 144 in a direction to increase the frequency of the reference oscillator 24 to thereby decrease the difference fre- I quency (IF) at the output of the mixer 28. A drift in the opposite direction (IF less than the normal frequency) would cause the final count of the binary counter 102 to be greater than eight, resulting in a low logic level on lead 106 and activation of the down counting input 124 of the counter 122.

It may be appreciatedfrom the foregoing that the frequency correction proceeds at a relatively slow rate of only one step or increment every five seconds until the IF signal is again at the normal frequency. Accordingly, the intermediate frequency may vary within the 500 Hz pass band of the filter 30 in response to gradual changes in the frequency of the loop oscillator 20 characteristic of changing environmental conditions, but the intermediate frequency will ultimately be returned to the normal frequency by the action of the automatic correction system.

Referring again to FIG. 3, it may be appreciated that the system provides for continuous monitoring of the intermediate frequency, in that the divide by six network 92 and the binary counter 102 are reset following a dead period by the leading edge 148 of the positive oing output of the inverter 96. This occurs when the output of the flip-flop again goes positive. Accordingly, the intermediate frequency is continuously monitored and compared with the standard or normal frequency, and the result of each comparison is stored in the register 108-110. The output state of the register is only sampled at 5 second intervals, however, as discussed above in order to purposely limit the rate at which a frequency correction is made.

From the foregoing, it may be appreciated that the correction system is incapable of compensating for a sudden variation in the intermediate frequency such as would be caused by a railway car entering the field of the loop 10. This shifts the intermediate frequency well outside of the 500 Hz pass band, even if the car is moving at a speed considerably slower than the slowest speed that would normally be encountered in practice. Furthermore, as has been previously discussed, the correction system is completely disabled once the intermediate frequency shifts outside of the pass band, since a loss of voltage at the output of the bridge rectifier 34 removes the power source from the pulse generator 66. Note in FIG. 2212 that the power leads 70 are connected with the unijunction transistor oscillator 116, which may be of the RC type where the voltage for the charging capacitor of the circuit is obtained directly from the leads 70.

Initial tune-up of the apparatus at the track site is readily accomplished utilizing the loop tuning capacitor 22 and a tuning switch 150 illustrated in FIG. 11. The tuning switch 150, upon closure thereof, is illustrated as connecting the up/down control 60 with the negative DC supply. (This is a purely functional illustration inasmuch as FIG. 1 is a simplified block diagram.) With the switch 150 closed, the incremental tuner 62 does not respond to the up/down control d but is, instead, preset at a step approximately midway in the incremental range. This step may, for example, be equated to the decimal number 75 or the seventy-fifth increment out of a total capacity of 160. Then with the movable pole 50 of the meter switch in the position illustrated in FIG. 1, the loop tuning capacitor 22 is adjusted until the meter 46 indicates that the IF signal is approximately centered in the pass band. The meter switch may then be returned to the off position, and the tuning switch 150 is opened. The automatic digital tuning system is now free to follow subsequent changes in the frequency of the loop oscillator as previously described.

Having thus described the invention, What is claimed as new and desired to be secured by Letters Patent is:

i. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, the improvement comprising:

output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions;

digital counting means responsive to said signal for deriving a count indicative of the frequency of said signal;

digital comparator means coupled with said counting means for comparing said count with a predetermined numerical value representing said normal frequency;

control means coupled with said comparator means and said variable frequency oscillator and responsive to a difference between said count and said value for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present; and

means responsive to said signal and coupled with said control means for disabling the latter to prevent operation thereof when the frequency of said signal is outside of said range, whereby to prevent correction of the frequency as long as the body is present in said zone.

2. The improvement as claimed in claim 1, wherein said counting means includes a first means for producing a train of pulses having a repetition rate indicative of the frequency of said signal, a second means responsive to said pulses for deriving a series of timing signals having a time duration from the beginning to the end of said series governed by the interval between a pair of said pulses, and a counter responsive to said series of timing signals for counting the latter and delivering said count to said comparator means.

3. The improvement as claimed in claim 1, wherein said counting means repeatedly delivers said count to said comparator means at successive intervals to monitor the frequency of said signal, said comparator means producing an output in response to each count which indicates whether the count is greater than, equal to, or less than said value, and wherein said control means includes a register for storing the output information from said comparator means, tuning means for increasing or decreasing the frequency of said oscillator, and means for actuating said tuning means in accordance with the information currently stored in said register if said information indicates that the frequency of said signal requires correction.

4. The improvement as claimed in claim 3, wherein said actuating means operates periodically to deliver said currently stored inforamtion to said tuning means whereby, whenever said information indicates that the frequency of said signal requires correction, the tuning means is periodically actuated until the frequency is corrected.

5. The improvement as claimed in claim 4, wherein the time period between successive operations of said actuating means is sufficiently long to effect said varying of the frequency of said oscillator at said slow rate.

6. The improvement as claimed in claim 5, wherein said oscillator has a circuit for controlling its frequency of operation, and wherein said tuning means includes a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components or a combination thereof I with said circuit, and means responsive to said stored information and coupled with said switching means for operating the latter to connect the component or com ponents with said circuit that will cause said signal to approach its normal frequency.

7. The improvement as claimed in claim 6, wherein said means responsive to said stored information in cludes an up/down counter having up and down counting inputs, and wherein said actuating means includes a pulse generator for producing time-spaced pulses defining the periodic operation of the actuating means, and an up/down control coupled between said register and said inputs and operable in response to each of said time-spaced pulses for exciting said inputs in accordance with said stored information.

8. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, the improvement comprising:

output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions, said output means including a band pass filter coupled with said detecting means and having a pass band defining said range;

frequency determining means responsive to said sig nal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency;

control means coupled with said frequency determining means and said variable frequency oscillator and responsive to a deviation of the frequency of said signal from said normal frequency for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present; and

means coupled with the output of said filter and with said control means for disabling the control means when the frequency of said signal is outside of said pass band, whereby to prevent cofrection of the frequency as long as the body is present in said zone.

9. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, wherein said oscillator has a circuit for controlling its frequency of operation, the improvement comprising:

output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions; frequency determining means responsive to said signal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency; and

control means coupled with said frequency determining means and said circuit and responsive to a deviation of the frequency of said signal from said normal frequency for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present,

said control means including tuning means having a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components or a combination thereof with said circuit, and means responsive to said output from the frequency determining means and coupled with said switching means for operating the latter to connect the component or components with said circuit that will cause said signal to approach its normal frequency.

10. The improvement as claimed in claim 9, wherein said control means further includes a register for storing the output information from said frequency determining means, and means for periodically actuating said tuning means in accordance with the information currently stored in said register if said information indicates that the frequency of said signal requires correction.

wherein said means for operating the switching means includes an up/down counter having up and down counting inputs, and wherein said actuating means includes a pulse generator for producing time-spaced pulses defining the periodic operation of the actuating means, and an up/down control coupled between said registerand said inputs and operable in response to each of said time-spaced pulses for exciting said inputs in accordance with said stored information.

12. The improvement as claimed in claim 10, wherein the time period between successive operations of said actuating means is sufficiently long to effect said varying of the frequency of said oscillator at said slow rate.

13. The improvement as claimed in claim 10, further comprising means responsive to said signal and coupled with said actuating means for disabling the latter to prevent operation of said tuning means when the frequency of said signal is outside of said range, whereby to prevent correction of the frequncy as long as the body is present in said zone.

14. In apparatus for determing whether a movable metallic body is present in a preselected zone of interll. The improvement as claimed in claim 10,

est along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including vari able frequency circuit means capable of controlling the frequency of said signal, the improvement comprising:

output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions;

frequency determining means responsive to said signal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency; and

control means coupled with said frequency determining means and said circuit means and responsive to a deviation of the frequency of said signal from said normal frequency for causing said circuit means to vary the signal frequency at a slow rate relative to the rate of change thereof when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present,

said control means including a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components of a combination thereof with said circuit means, and means responsive to said output from the frequency determining means and coupled with said switching means for operating the latter to connect the component or components with said circuit means that will cause said signal to approach its normal frequency.

15. The improvement as claimed in claim 14, wherein said means for operating the switching means includes a plurality of digital control connections operably coupled with said switching means, and a counter whose output comprises said digital control connections and which has input means receiving the output information from said frequency determining means.

16. The improvement as claimed in claim 15., wherein said control means further includes means for periodically delivering said output information to said input means of the counter at time intervals of sufficient length to provide said slow rate of variation of the signal frequency. 

1. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, the improvement comprising: output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions; digital counting means responsive to said signal for deriving a count indicative of the frequency of said signal; digital comparator means coupled with said counting means for comparing said count with a predetermined numerical value representing said normal frequency; control means coupled with said comparator means and said variable frequency oscillator and responsive to a difference between said count and said value for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present; and means responsive to said signal and coupled with said control means for disabling the latter to prevent operation thereof when the frequency of said signal is outside of said range, whereby to prevent correction of the frequency as long as the body is present in said zone.
 2. The improvement as claimed in claim 1, wherein said counting means includes a first means for producing a train of pulses having a repetition rate indicative of the frequency of said signal, a second means responsive to said pulses for deriving a series of timing signals having a time duration from the beginning to the end of said series governed by the interval between a pair of said pulses, and a counter responsive to said series of timing signals for counting the latter and delivering said count to said comparator means.
 3. The improvement as claimed in claim 1, wherein said counting means repeatedly delivers said count to said comparator means at successive intervals to monitor the frequency of said signal, said comparator means producing an output in response to each count which indicates whether the count is greater than, equal to, or less than said value, and wherein said control means includes a register for storing the output information from said comparator means, tuning means for increasing or Decreasing the frequency of said oscillator, and means for actuating said tuning means in accordance with the information currently stored in said register if said information indicates that the frequency of said signal requires correction.
 4. The improvement as claimed in claim 3, wherein said actuating means operates periodically to deliver said currently stored inforamtion to said tuning means whereby, whenever said information indicates that the frequency of said signal requires correction, the tuning means is periodically actuated until the frequency is corrected.
 5. The improvement as claimed in claim 4, wherein the time period between successive operations of said actuating means is sufficiently long to effect said varying of the frequency of said oscillator at said slow rate.
 6. The improvement as claimed in claim 5, wherein said oscillator has a circuit for controlling its frequency of operation, and wherein said tuning means includes a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components or a combination thereof with said circuit, and means responsive to said stored information and coupled with said switching means for operating the latter to connect the component or components with said circuit that will cause said signal to approach its normal frequency.
 7. The improvement as claimed in claim 6, wherein said means responsive to said stored information includes an up/down counter having up and down counting inputs, and wherein said actuating means includes a pulse generator for producing time-spaced pulses defining the periodic operation of the actuating means, and an up/down control coupled between said register and said inputs and operable in response to each of said time-spaced pulses for exciting said inputs in accordance with said stored information.
 8. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, the improvement comprising: output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions, said output means including a band pass filter coupled with said detecting means and having a pass band defining said range; frequency determining means responsive to said signal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency; control means coupled with said frequency determining means and said variable frequency oscillator and responsive to a deviation of the frequency of said signal from said normal frequency for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present; and means coupled with the output of said filter and with said control means for disabling the control means when the frequency of said signal is outside of said pass band, whereby to prevent correction of the frequency as long as the body is present in said zone.
 9. In apparatus for determining whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including a variable frequency oscillator capable of controlling the frequency of said signal, wherein said oscillator has a circuit for controlling its frequency of operation, the improvement comprising: output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions; frequency determining means responsive to said signal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency; and control means coupled with said frequency determining means and said circuit and responsive to a deviation of the frequency of said signal from said normal frequency for varying the frequency of said oscillator at a slow rate relative to the rate of change of the frequency of said signal when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present, said control means including tuning means having a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components or a combination thereof with said circuit, and means responsive to said output from the frequency determining means and coupled with said switching means for operating the latter to connect the component or components with said circuit that will cause said signal to approach its normal frequency.
 10. The improvement as claimed in claim 9, wherein said control means further includes a register for storing the output information from said frequency determining means, and means for periodically actuating said tuning means in accordance with the information currently stored in said register if said information indicates that the frequency of said signal requires correction.
 11. The improvement as claimed in claim 10, wherein said means for operating the switching means includes an up/down counter having up and down counting inputs, and wherein said actuating means includes a pulse generator for producing time-spaced pulses defining the periodic operation of the actuating means, and an up/down control coupled between said register and said inputs and operable in response to each of said time-spaced pulses for exciting said inputs in accordance with said stored information.
 12. The improvement as claimed in claim 10, wherein the time period between successive operations of said actuating means is sufficiently long to effect said varying of the frequency of said oscillator at said slow rate.
 13. The improvement as claimed in claim 10, further comprising means responsive to said signal and coupled with said actuating means for disabling the latter to prevent operation of said tuning means when the frequency of said signal is outside of said range, whereby to prevent correction of the frequncy as long as the body is present in said zone.
 14. In apparatus for determing whether a movable metallic body is present in a preselected zone of interest along a path of travel thereof, where a sensing element is adapted for disposition adjacent said zone and detecting means is employed which is coupled with said element for generating an oscillatory electrical signal having a frequency subject to relatively slow variation in accordance with changing environmental conditions in said zone and sudden variation as said body passes through said zone, said detecting means including variable frequency circuit means capable of controlling the frequency of said signal, the improvement comprising: output means responsive to said signal and having an output condition indicating that said body is present in said zone when the frequency of said signal is outside of a predetermined range within which the signal is permitted to vary from a normal frequency in response to changing environmental conditions; frequency determining means responsive to said signal for producing an output which indicates whether the frequency thereof is greater than, equal to, or less than said normal frequency; and control means coupled with said frequency determining means and said circuit means and responsive to a deviation of the frequency of said signal from said normal frequency for causing said circuit means to vary the signal frequency at a slow rate relative to the rate of change thereof when said body passes through said zone, whereby to correct the frequency of said signal in response to changing environmental conditions to prevent a false indication of the presence of the body, without preventing said output means from responding when the body is actually present, said control means including a plurality of frequency varying components, electrically responsive switching means for operably connecting any of said components of a combination thereof with said circuit means, and means responsive to said output from the frequency determining means and coupled with said switching means for operating the latter to connect the component or components with said circuit means that will cause said signal to approach its normal frequency.
 15. The improvement as claimed in claim 14, wherein said means for operating the switching means includes a plurality of digital control connections operably coupled with said switching means, and a counter whose output comprises said digital control connections and which has input means receiving the output information from said frequency determining means.
 16. The improvement as claimed in claim 15, wherein said control means further includes means for periodically delivering said output information to said input means of the counter at time intervals of sufficient length to provide said slow rate of variation of the signal frequency. 